Question

From section (6.2.2/7) C99 Standard

7. If, within a translation unit, the same identifier appears with
both internal and external linkage, the behavior is undefined.

While the following generates a compile time error due to conflict of definitions

// 'x' has external linkage
extern int x;

// Here, 'x' has internal linkage
static int x;

But the following compiles fine,

// 'x' has external linkage
extern int x;

void foo() {
  // Here, 'x' has internal linkage
  static int x;
}

Do both the cases invoke an undefined behavior?

Answer 1

Your question stems from an incorrect assumption that a locally declared static variable has internal linkage. In reality a static variable declared in a block scope has no linkage. See 6.2.2/6

6 The following identifiers have no linkage: an identifier declared to
be anything other than an object or a function; an identifier declared
to be a function parameter; a block scope identifier for an object
declared without the storage-class specifier extern.

Only file-scope declaration can have external or internal linkage (plus local extern declarations).

Therefore 6.2.2/7 and your question simply do not apply.